Process for minimizing ridging in chromium steels



United States Patent ()fifice 3,128,211 Patented Apr. 7, 1964 3,128,211 PRGCESS FOR MINIMIZING RIDGING 1N CHROMIUM STEELS James H. Waxweiler, Middietown, Ohio, assignor to Armco Steel Corporation, Middletowu, Ohio, at corporation of Ohio No Drawing. Filed Aug. 14, 1961, Ser. No. 131,092 Claims. (Cl. 148-12) As is well known, ferritic chromium steels, particularly those responding generally to A.I.S.I. Type 430, are subject, when drawn, to a defect known as ridging or roping. This defect manifests itself in the formation of ridges in the metal parallel to the direction in which the metal was originally rolled, when the metal is drawn. The ridges make the drawn article unsightly and lead to rejections, especially since there is no way of restoring the appearance of a ridged article excepting by an expensive grinding operation; and even this is not always effective.

The skilled worker in the art will understand that the behavior of chromium bearing steels, so far as the ridging tendency is concerned, has for the most part been erratic and unpredictable, and also that there are degrees of ridging or roping; some products, though characterized by a detectable amount of ridging being nevertheless suitable for some uses. Other uses may require a much higher resistance to ridging in the chromium bearing steel sheet material. Consequently there is substantial utility in a procedure which will minimize ridging, since the effect of such a procedure may be to permit the use of the sheet material in a particular field which it could not enter otherwise. At the same time, an ideal sheet material in accordance with this invention may be considered one which, when subjected to a reasonable depth of draw, will either not present visually any appearance of ridging, or a drawn material in which any appearance of striations will be destroyed by a simple bufi'lng operation, not intended for the removal of appreciable metal. For convenience, the term non-ridging will be used herein, and is intended to embrace not only materials which will present no visual appearance of ridging whatever when drawn, but also those materials in which the degree of ridging, with or without buifing, is insufficient to render them unsuitable in the uses for which they are intended.

As yet there is no test giving a quantitative measurement of ridging. It has been found that if a strip of the chromium bearing steel is clamped at its ends in a suitable machine and then stretched to a point near but just short of its breaking point, observable ridging will occur in the stretched portion if the metal has any ridging tendency at all. But the degree of ridging must be judged by the eye, so that the test results are, to this extent, subjective.

it is a fundamental object of the invention to provide procedures which, applied to chromium bearing steels as hereinafter set forth, can be depended upon to produce a sheet material exhibiting a useful diminution of ridging, and under optimum conditions to produce the ideal material above defined.'

It is an object of the invention to provide procedures for the purpose set forth, which do not add appreciably to the cost of production of the material.

It is also an object of the invention to provide procedures which yield a material of physical characteristics suitable for deep drawing while having the diminished roping tendency or the absence of roping characteristics to which reference has been made.

These and other objects of the invention which will be set forth hereinafter in detail, or will be apparent to one skilled in the art upon reading these specifications, are accomplished by the employment of the processing steps and controls of which exemplary embodiments will now be described.

Over the years, a great deal of Work has been done on the ridging problem. For example, on the theory that the condition of carbon or carbides in the final product has a controlling effect on ridging, proposals have been made looking toward the addition of carbide formers in small amounts, inclusive of tungsten, vanadium, titanium, columbium, and perhaps others, to the steel. It is entirely conceivable that under some particular circumstances the addition of such an alloying ingredient might effect a useful diminution of ridging; but the particular circumstances in which this could occur have not been recognized, understood or defined; and it is not true that the bare addition of a carbide-former to the steel will, in and of itself, bring about either a diminution or an elimination of the ridging tendency.

US. Patent No. 2,851,384 to the present inventor taught the minimization of ridging by a procedure involving the chemical control of the austenite potential of the metal, and a series of steps following a hot rolling, which steps include at least one heat treatment above the critical temperature. This method was successful in minimizing ridging; but it involved certain aspects of expense, and was characterized by the frequent occurrence of relatively high hardness and reduced formability of the strip. Consequently there Was need for a less expensive process capable of producing a chromium steel sheet which had better drawing characteristics.

The exact mechanism which is responsible for ridging is not accurately defined; but research has now shown that chromium steel sheet materials of the general classification of A.I.S.l. Type 430 may be made readily drawable and devoid of ridging or showing low ridging tendencies by procedures involving control of the grain characteristics of the metal. The factors having useful effects upon the attainment of the desired grain characteristics may be listed as:

(1) The chemistry of the metal,

(2) The production of ingots having certain qualities,

(3) The manner in which the hot rolling is carried on,

and

(4) The nature of the steps following the hot rolling.

These factors will now be discussed under appropriate headings, in which discussion they will be subdivided. The various factors are capable of coacting and do coact in any routing in which two or more of them are employed to diminish the tendency for the metal to ridge or rope. This does not mean that all of the recommended factors need be employed in any given routing. In listing the factors above, they have been arranged in the order in which they can be practiced, without any attempt at arrangement in the order of importance. The interrelation of the factors will be hereinafter described.

CHEMISTRY OF THE METAL The metal itself will generally respond to classification Type 430 of the American Iron and Steel Institute, which classification can be found in Steel Products Manual on Stainless and Heat Resisting Steels published by A.I.S.I. Metal of this classification is sometimes referred to as 17-chrome steel" because it contains about that percentage of chromium and is devoid of or has only a nugatory quantity of nickel. It is such a metal that presents the ridging or roping tendency to which this invention is directed.

The A. I.S.I. Type 430 limits for a l7-chrome steel are:

Percent Cr 14.00-18.00 C, maxim-um .12 Mn, maximum 1.0 Si, maximum 1.0 P, maximum .040 S, maximum .030

Molybdenum may be added up to about 2% for corrosion resistance without aifecting the results produced by the present invention.

The quantities of the elements listed above can be followed as usual in the manufacture of a Type 430 steel. It is not necessary, as in Patent 2,851,384, to control the austenite potential of the material. Whereas in that patent a higher austenite potential than usual for a Type 430 heat was used, the austenite potential in the metal of the present case is likely to be lower than usual for such a heat.

It has been found in the practice of the present invention that it is well to keep the carbon content relatively low, i.e., to a maximum of .06%, with a preferable maximum for carbon at .045

For reasons which will be set forth hereinafter, it is preferred to add to the metal a very small percentage of a material which will tend to affect the nature of the grains in the ingot and in the hot rolled intermediate gauge product. The most effective material has been found to be columbium, which is preferably used in a quantity ranging from .05 to 50% by weight of the metal. More may be used up to 1% if desired; but the added expense is not generally justified. The columbium may be added in the form of ferro-columbium, and, despite the fact that ferro-columbium is frequently contaminated with a small amount of tantalum, the presence of the tantalum may be disregarded, and the quantity of material calculated as though it were ferro-columbium alone.

The research on which this invention is based has not shown that columbium or any other similar element has a direct effect on the ridging or roping tendency of the Type 430 metal. This is to say that if a particular routing tends to produce a Type 430 metal which is characterized strongly by ridging or roping tendencies, the bare addition of columbium or the like will not generally produce a diminution of the tendencies. Also it is entirely possible to produce non-ridging metal without columbium. But an indirect effect is obtained in that columbium helps in the attainment of the desired final grain condition, so that if the features of the routing are such as to tend toward the production of such grain condition, the addition material makes for more uniform production and hence on the whole, reduced ridging.

THE PRODUCTION OF INGOT When Type 430 metal is cast in the usual ingot mold in accordance with standard procedure in the art, the ingot tends to exhibit columnar grain structure. It has been found, in accordance with this invention, that the attainment of low ridging structure in the sheet stock is promoted by the formation of ingots with an equiaxed grain structure, preferably of a small grain size, as distinguished from the familiar dendritic or columnar structure. There are various degrees of departure from a coarse dendritic structure which can be accomplished by known expedients in the formation of the ingot. One of these expedients is cold teeming, i.e., pouring the molten metal into the ingot mold with as small an amount of superheat as is feasible. Other common expedients include the vibration of the ingot mold during the solidification of the metal, at various rates extending up to and including rates of vibration within the supersonic range. Yet another expedient is the addition to the metal before or after teeming of some chemical element or compound which will have a seeding effect tending toward small grain nucleation or will cool the melt and eliminate superheat. Any or all of these expedients may be employed.

The formation of ingots characterized by an equiaxed grain structure of small grain size has this importance: that it can make the difference between high and low ridging behavior in metals of the same composition. Putting this another way, it is quite possible to achieve low ridging characteristics in the process of this invention without controlling the hot rolling as set forth herein under the next heading, providing equiaxed, very fine grain ingots are initially produced. The skilled worker in the art will recognize that the ultimate in ingot control may be costly; but it is worth-while to take any of the above precautions in ingot making which are found to be commercially feasible, especially since the attainment of an equiaxed fine grain condition in the ingots coacts with the control of the hot rolling as next described.

THE HOT ROLLING While the production of ingots in which the grain structure tends to be small and equiaxed is most helpful in obtaining the final grain condition to which this invention is addressed, a control of the hot rolling procedure is, in present commercial practice, very nearly essential, in view of cost considerations. Even though the ingot may have a dendritic structure, it is possible to produce the desired kind of hot rolled intermediate gauge product by various operations which may be summarized in the term low temperature hot rolling. In theory it would be possible to carry an ingot down to the desired intermediate gauge and attain the desired grain structure by a series of operations, all of which are practiced at a low temperature as hereinafter defined. However, the skilled worker in the art will understand that the common methods of hot rolling ingots to intermediate gauge sheet material contemplate the use of various forms of continuous hot mill trains, and that the feasibility of securing the desired intermediate gauge in the operation will be based upon obtaining the desired percentage reduction in the totality of the mill stands which, in turn, will require that a considerable part of the hot rolling be carried on at a relatively high temperature.

'In general there are two commercially recognized processes by which an ingot may be carried down to an intermediate hot rolled gauge. In one of these processes the ingot itself or blooms made therefrom are heated to a relatively high temperature, and the hot rolling or subsequent hot rolling is done without reheating. In a more common process the ingots, after heating in the soaking pits, are rolled into slabs which may vary as to thickness but which in an exemplary procedure are 2 in. to 6 in. thick. These slabs are reheated in a slab heating furnace and then rolled down to the intermediate gauge in a continuous hot mill. The principles of the present invention are applicable to either of these processes.

To obtain the grain quality desired, it is necessary that at least a substantial final hot rolling be practiced on the product at a temperature not higher than about 1500 to 1600 F. While some variation is tolerable, the material being hot rolled should attain a temperature not exceeding the above range at a point at which a reduction of at least about 50% remains necessary in order to reach the desired intermediate gauge. In the performance of the hot rolling it is difficult to control the temperature of the material especially at the first or roughing mill pass. Nevertheless, the objects of this invention maybe attained with regularity in a hot rolling procedure which includes the formation of slabs and the reheating of the slabs for further reduction, by lowering the temperature to which the slabs are reheated. Thus, While in standard practice the slabs are usually heated to a temperature of about 2050" R, if the reheating temperature is lowered so as to lie in the range of substantially 1800 to 1950 F. or thereabouts, the desired grain condition will be attained after rolling. It will be understood that the final temperature of the hot rolled material will depend to some extent upon the nature of the tandem train of hot mills or other apparatus by which the reduction is made and that, therefore, some variation in the relatively low reheating temperature range may be made.

A more positive procedure involves interrupting the hot rolling at a point where at least about a 50% reduction in thickness remains to be accomplished, and cooling the material during the interruption to a temperature range of about 1400 to 1600 F. This may be accomplished by tabling the material until it has cooled as set forth, the temperature being determined by temperature indicating means. Optical or other pyrometers may be used, as well as thermocouples, and other suitable means.

It will be understood that where necessary in order to interrupt the hot rolling, the partially hot rolled material may be sheared into pieces of suitable size for tabling. In a conventional hot rolling operation in which slabs are formed and reheated, the tabling and cooling of the material can frequently be done between the roughing stands and the finishing stands of the hot mill assembly.

That there is a distinct grain quality which should be attained, is evident from the fact that the nature of the hot rolling makes a very great difference in the ridging or roping characteristics of the final product. In many instances, the difference will lie in a marked decrease in ridging tendencies where the hot rolling has been carried on as set forth above. Assuming proper cold rolling and annealing procedures as hereinafter taught, the nature of the hot rolling has been found to be the most important variable in the routing, using normal dendritio ingots.

The formation of ingots having a fine equiaxed grain structure is perhaps the most positive method of ensuring low ridging or low roping characteristics in the steel sheet or strip, but this formation is difficult to attain in the very large ingot sizes normally used in production of Type 430. Thus in considering these large ingots, one is faced with normal ingots having large dendrites, or ingots which have been treated in a manner to produce some equiaxed structure, but which also contain some dendritic pattern. With these partially or fully dendritic ingots, it is still possible to make non-ridging material or material having low ridging quality, providing the hot rolling is accomplished, at least in the final stages, at temperatures of 1400 to 1600 F. or below, and as above described.

The skilled worker in the art will understand that the presence of a marked dendritic character in the ingot may require certain precautions in the initial stages of the hot rolling to avoid disruption or cracking of the metal. A portion of the hot rolling for reasons of rolling efficiency will be carried on at a high temperature, say, in the neighborhood of 2000 F.; and so long asthe material is capable of being hot rolled, the carrying on of the finishing stages of the hot rolling at temperatures which will lie within or below the range of about 1400 to 1600 F. during a final reduction of 50% or-more will produce a grain condition such as to minimize ridging.

The specific hot rolled intermediate gauge is not a limitation on the invention, since it will be followed by cold rolling treatments to reduce the material to the final gauge. Thus, the intermediate hot rolled gauge may be varied in the light of the cold rolling treatment or treatments and the desired final gauge. An intermediate hot rolled thickness of about .1 in. to .3 in. may be regarded as exemplary but not limiting.

THE NATURE OF THE STEPS FOLLOWING THE HOT ROLLING The intermediate gauge hot rolled material is preferably subjected to a box anneal at a temperature of about 1500 F. A lower temperature such as 1450 F. or less has not been found to be advantageous. Since the grain characteristics of the hot rolled material are largely determinative of the ridging behavior of the final product, a heat treatment which would carry the material to a temperature above the critical is not necessary. The hot rolled material may be box annealed in coil or sheet form; and control of the annealing atmosphere is permissible but not necessary. An open or continuous short time strand anneal of the hot rolled structure has not been found to be as eifective as a box anneal. Thus, the preferred heat treatment following hot rolling may be stated to be a box anneal at a temperature substantially within the range of 1475 to 1525 F., but not above the critical.

The box annealed product is then descaled. Thereafter the metal is cold reduced and then subjected to an open or continuous strand anneal at about 1500 F. Preferably, the cold reduction is accomplished in two or more stages with an intermediate anneal which is also an open or continuous strand anneal at about 1500 F. It has been determined that one or more intermediate anneals between the hot rolled thickness and the final gauge of the material is helpful in obtaining low ridging characteristics of the stock providing prior operations have been performed as taught. When the total cold reduction required is over 50%, a material formed by cold reducing the box annealed product in a single stage and without any intermediate anneal behaves erratically and is frequently found to exhibit no improvement in ridging characteristics. However, when the box annealed stock is carried down to final gauge, by at least two cold rolling treatments with an intermediate anneal or anneals, the desired effect is obtained. Two or three intermediate annealing steps are optimum for the purpose of this invention, but are not usually practiced because of the expense. In general two stages of cold rolling with a single intermediate anneal and a single final anneal will be found satisfactory, and for reasons of economy constitutes standard practice in accordance with the present invention.

The cold rolling reduction should be at least about 50% total, and may be as much as or more than 50% per stage. A hot rolled material having a thickness of about .170 in. may be cold rolled to about .080 in., while a hot rolled material having a thickness of about .200 in. may be cold rolled to about .110 in. in a first cold rolling stage. These figures are exemplary but not limiting.

A further reduction in gauge in a second stage of cold rolling may be made following the intermediate anneal. Preferably, the hot rolled stock will be reduced to the final sheet gauge in .two stages of cold rolling, each of which eifects a reduction of at least about 50%. When the final thickness of the stock is to be .025 in. or less, and where extra deep drawing and exceptionally low ridging qualities are requirements, it is recommended that three stages of cold rolling be employed with two intermediate annealing treatments. The point in the routing at which the second intermediate anneal occurs is best selected in such a way that about a 50% reduction will occur in the final cold rolling stage.

It has been found that where three cold rolling stages are employed with two intermediate anneals, the extra intermediate anneal tends to provide lower ridging as well as higher drawability.

The intermediate anneal or anneals are open or continuous anneals at temperatures substantially between 1450 and 1550 F., but not above the critical. The anneals may be carried on in an air atmosphere; but in this event each such open anneal will be followed by a pickling treatment to descale the stock. Bright annealing practices may be employed for the intermediate anneals if desired; but it is generally cheaper to anneal in a furnace open to the air and then pickle.

The final annealing treatment will again be an open or strand anneal within the same temperature range, namely from about 1450" to about 1550 F. but not have the critical. It is preferred that the final strand anneal be carried on under bright annealing conditions, that is to say, in a neutral or reducing atmosphere. This is especially valuable where the final product can benefit from the use of the bright dense surface finish obtained with a bright anneal. However, for some uses the final anneal may be carried on in an air atmosphere with the product thereafter being pickled.

The cold rolling may be carried on with any type of cold rolling equipment, including but without limitation, tandem trains of cold mills where the product is in strip form, 4-high single strand reducing mills, or Sendzimir cluster mills. There is evidence which indicates that some what better ridging grades may be attained by the use of accurate cold mills having small highly polished Working rolls and acting to produce a flat product, i.e., one not charcterized by substantial gauge variation from one side of the strip to the other. A particular mill of this type, Well known in the art, is the Sendzimir cold mill.

Following the final anneal, a temper rolling producing from a fractional percentage to a few percent reduction may be used if desired for the elimination of the yield point elongation and control of the drawing characteristics of the product.

In the open annealing treatments'which constitute the intermediate and final anneals, there are no essential time limitations, it being necessary only to bring the meterial to the temperature range indicated under such conditions that all portions of the material will reach the desired temperature before coling ensues. Without limitation, three minutes per .100 inch of thickness will be adequate.

The final cold rolled thickness for A.I.S.I. Type 430 chromium steels in which drawing qualities are desired will lie generally between about 0.15 in. to .060 in.

The result of the cold rolling and annealing treatments hereinabove outline, where a proper grain condition has been attained in the intermediate hot rolled product, is the production of a steel which is relatively fine grained and which also is characterized by an absence or minimum amount of ridging or roping.

Example I A chromium steel responding to A.I.S.I. Type 430 and containing about .04% carbon and about .10% columbium was hot rolled under conditions such that least a final hot rolling reduction of about 50% was eifected at a temperature not exceeding the range of 1400 to 1600 F. and processed to a finished coil by the following routing:

Hot roll to a thickness of .170 in. Box anneal at 1500 F.

Pickle.

Cold reduce to .080 in.

Open anneal at 1500 F.

Pickle.

(7) Cold reduce to .025 in.

(8) Anneal-descale if necessary.

(9) Temper roll.

It was found that the use of ingots having a fine grained equiaxed structure improved the ridging behavior of the product as did variations inthe amount of fin al hot rolling reduction carried on at temperatures Within the range set forth or lower temperatures down to about 11350" F. under the best operating conditions it was found that a low ridging grade could be attained without the use of columbium.

Exampl 11 The same metal as used in Example I, excepting that it contained no columbium and only 0.3% carbon, was formed into an ingot having an equiaxed small grain structure and then subjected to a hot rolling treatment in which the product was tabled and cooled to a temperature not greater than 1500 to 1600 F. by the optical pyrometer, after which a further reduction of at least 50% was made in it by hot rolling. The routing was as follows:

(1) Hot roll to .170 in. (2) Box anneal.

(3) Pickle.

(4) Cold reduce to .080 in. (5) Open anneal.

(6) Pickle.

(7) Cold reduce to .050 in. (8) Open anneal.

(9) Pickle.

(10) Cold reduce to .025 in. l1) Anneal-descale if necessary. (12) Temper roll.

This product exhibited low ridging in drawn articles made from it, by which is meant that when the product was stretched or drawn, it exhibited no observable ridging. Like austenitic stainless steel containing 18% chromium and 8% nickel, the surface of the sheet material was just as smooth after drawing or stretching as before.

Modifications may be made in the invention without departing from the spirit of it. The invention having been described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

l. A process of diminishing ridging upon drawing in ferritic chromium-bearing steels having a chromium content of about 14.00% to 20% and not more than a nugatory quantity of nickel, which process comprises hot rolling the steel to an intermediate gauge, with at least the final 50% of the hot rolling reduction taking place at a temperature below 1600 F., box annealing the hot rolled product, and cold rolling it to gauge with a reduction of at least about 50% in at least two cold rolling stages, each followed by an anneal, the said anneals being at temperatures below the critical temperature, whereby to produce a product having a fine grain size and exhibiting low ridging characteristics.

2. The process claimed in claim 1 in which the steel has a carbon content not greater than about .02% to .05

3. The process claimed in claim 1 in which the steel has a carbon content not greater than about .02% to .05%, and a content of substantially .05% to substantially .50% of columbium.

4. The process claimed in claim 1 in which the steel has a carbon content not greater than about .02% to .05 and contains from about .10% to about .25 columbium.

5. The process claimed in claim 1 including the step of forming steel into ingots preparatory to hot rolling, in which ingots a dendritic structure is minimized and the grains tend to be small and equiaxed.

6. The process claimed in claim 1 wherein the cold rolling is divided into at least two stages with an intermediate anneal between the stages, any intermediate anneals being open annealing treatments at a temperature substantially within the range of l450 to 1550 F.

7. The process claimed in claim 1 wherein the cold rolling is divided into at least two stages with an intermediate anneal between the stages, said intermediate anneal being an open annealing treatment at a temperature substantially within the range of 1450 to 1550 F., the final one of said cold rolling stages effecting a reduction of at least about 50% in the metal.

8. The process claimed in claim 2 wherein the metal is formed into ingots preparatory to hot rolling, with precautions to minimize the formation of a coarse dendritic structure therein.

9. The process claimed in claim 2 wherein the metal is formed into ingots preparatory to hot rolling, with precautions to minimize the formation of a coarse dendritic structure therein, and in which the cold rolling is carried on in at least two stages with an intermediate anneal at a temperature of substantially 1450 to 1550 F.

10. The process claimed in claim 2 wherein the metal is formed into ingots preparatory to hot rolling, with precautions to minimize the formation of a coarse dendritic structure therein, and in which the cold rolling is carried on in at least two stages with an intermediate anneal at a temperature of substantially 1450 to 1550 F., and in which the final one of said cold rolling stages elfects a reduction in the metal of at least about 50%.

11. The process claimed in claim 3 including the step of initially forming the metal into ingots with precautions to minimize the production of a coarse dendritic crystal structure.

121 The process claimed in claim 3 including the step of initially forming the metal into ingots with PIGCEUIJti'OIIS to minimize the production of a coarse dendritic crystal structure, and in which the cold rolling is carried on in at least two stages with an intermediate anneal at a temperature of substantially 1450 to 1550" F.

13. The process claimed in claim 3 including the step of initially forming the metal into ingots with precautions to minimize the production of a coarse dendritic crystal structure, and in which the cold rolling is carried on in at least two stages with an intermediate anneal at a tem- 4 perature of substantially 1450" to 1550 F., and in which 10 the final one of said cold rolling stages efiects a reduction in the metal of at least substantially 14. A process of dimenishing ridging upon drawing in ferritic chromium-bearing steels corresponding to A.I.S.I Type 4 30, said steels containing less than about 05 carbon and substantially .10% to .25 columbium, which comprises hot rolling the metal to an intermediate gauge, interrupting the hot rolling at a point in the reduct-ion of the metal such that at least about 50% hot rolling remains to be carried on to reduce the metal to the said intermediate gauge, and while the hot rolling is interrupted, cooling the partially hot rolled material to a temperature below about -'l600 -F. and thereupon contin-uing the hot rolling to the said intermediate gauge, box annealing the hot rolled product at a temperature of substantially 1475 to 1525 F., cold rolling the product to the desired final gauge in at least two cold rolling stages, the last of which efiects a cold rolling reduction of at least about 50%, subjecting the product between the cold rolling stages to open annealing treatments at a temperature of substantially 1450" to 1550 F., and after the last cold rolling stage, subjecting the product to an open annealing treatment at a temperature of substantially-1450 to 1550 F., whereby to produce a product of fine grain size.

15. A process of diminishing ridging upon drawing in ferritic chromium-bearing steels having a chromium content of about 14% to 20% and no more than a nugatory amount of nickel, which process comprises hot rolling the steel to an intermediate gauge, with at least the final 50% of the hot rolling reduction taking place at a temperature below 1600 F., box annealing the hot rolled product, and cold rolling it to gauge with at least a final anneal at a temperature below the critical temperature.

References Cited in the file of this patent UNITED STATES PATENTS 1,980,331 George Nov. 13, 1934 2,340,461 Gage et al. Feb. 1, 1944 2,808,353 Lefiingwell Oct. 1, 1957 

1. A PROCESS OF DIMINISHING RIDGING UPON DRAWING IN FERRITIC CHROMIUM-BEARING STEELS HAVING A CHROMIUM CONTENT OF ABOUT 14.00% TO 20% AND NOT MORE THAN A NUGATORY QUANTITY OF NICKEL, WHICH PROCESS COMPRISES HOT ROLLING THE STEEL TO AN INTERMEDIATE GAUGE, WITH AT LEAST THE FINAL 50% OF THE HOT ROLLING REDUCTION TAKING PLACE AT A TEMPERATURE BELOW 1600*F., BOX ANNEALING THE HOT ROLLED PRODUCT, AND COLD ROLLING IT TO GAUGE WITH A REDUCTION OF AT LEAST ABOUT 50% IN AT LEAST TWO COLD ROLLING STAGES, EACH FOLLOWED BY AN ANNEAL, THE SAID ANNEALS BEING AT TEMPERATURES BELOW THE CRITICAL TEMPERATURE, WHEREBY TO PRODUCE A PRODUCT HAVING A FINE GRAIN SIZE AND EXHIBITING LOW RIDGING CHARACTERISTICS. 